1. Field of the Invention
The present invention relates to a magnetic reproducing device with a rotary head drum, such as a video tape recorder, a digital audio tape recorder, or the like.
2. Description of the Prior Art
There are known helical-scan magnetic recording devices employing rotary head drums, such as video tape recorders (VTR), digital audio tape recorders (DAT), or the like.
The magnetic tape in such magnetic recording devices can usually be transported selectively at different speeds so that information can be recorded on the magnetic tape in different tape speed modes. When the tape speeds are changed to reproduce the recorded information in one of the tape speed modes, the speed at which the rotary head drum rotates is also varied so that the recorded information can properly be reproduced at the selected tape speed.
It is often customary to record auxiliary information such as address signals as well as main information such as video signals on magnetic tapes which are played back by helical-scan magnetic recording devices. Typically, as shown in FIG. 3 of the accompanying drawings, pieces of auxiliary information D are recorded in predetermined areas in successive inclined tracks T1, T2, T3, . . . on a magnetic tape by a rotary magnetic head on a rotary head drum.
When the recorded information is to be reproduced, the timing for a playback head to scan the areas where the pieces of auxiliary information D are recorded is detected, and the pieces of auxiliary information D are extracted with the detected timing. More specifically, switching pulses are generated in synchronism with rotation of the rotary head drum, as shown in FIG. 4A, each switching pulse being produced in one revolution of the rotary head drum. If the rotary head drum has two magnetic heads, then one of the magnetic heads produces a reproduced signal as shown in FIG. 4B. The reproduced signal contains the pieces of auxiliary information D at spaced time intervals. Window pulses shown in FIG. 4C are produced in timed relation to the pieces of auxiliary information D, and the pieces of auxiliary information D are extracted from the reproduced signal by the window pulses.
When the rotational speed of the rotary head drum is varied for the reproduction of the recorded information in another tape speed mode, it is necessary to vary the window pulses in timed relation to the rotational speed of the rotary head drum, as shown in FIGS. 5a-5C. Specifically, FIG. 5C shows window pulses which are produced when the rotational speed of the rotary head drum is about 1.5 times higher the normal rotational speed as shown in FIG. 4A. The magnetic head produces a reproduced signal as shown in FIG. 5B in timed relation to switching pulses as shown in FIG. 5A. At this time, the period of time which is required for the magnetic head to scan each track on the magnetic tape is shorter, as shown in FIG. 5B, than when the rotary head drum rotates at the normal rotational speed. The recorded pieces of auxiliary information D cannot be extracted unless the window pulses are produced in synchronism with the reproduced signal, as shown in FIG. 5C.
However, a complex circuit arrangement is needed to vary the window pulses in timed relation to the rotational speed of the rotary head drum. Thus, it has proven difficult to extract necessary auxiliary information from a reproduced signal when the rotational speed of the rotary head drum is varied.